Alternative lighting devices to replace incandescent light bulbs differ from incandescent light bulbs in the manner that energy is converted to light. Incandescent light bulbs include a metal filament. When electricity is applied to the metal filament, the metal filament heats and glows, radiating light into the surrounding area. The metal filament of conventional incandescent light bulbs generally has no specific power requirements. That is, any voltage and any current may be applied to the metal filament, because the metal filament is a passive device. Although the voltage and current need to be sufficient to heat the metal filament to a glowing state, any other characteristics of the delivered energy to the metal filament do not affect operation of the incandescent light bulb. Thus, conventional line voltages in most residences and commercial buildings are sufficient for operation of the incandescent bulb.
However, alternative lighting devices, such as compact fluorescent light (CFL) bulbs and light emitting diode (LED)-based bulbs, contain active elements that interact with the energy supply to the light bulb. These alternative devices are desirable for their reduced energy consumption, but the alternative devices have specific requirements for the energy delivered to the bulb. For example, the alternative devices may include controller integrated circuits (controller ICs) for controlling delivery of power to the LEDs, ballast, or other components of the bulb. The controller ICs are low-voltage devices built from similar components and with similar manufacturing techniques as computer devices, which also operate at low-voltages. However, whereas a computer device may have a bundled AC/DC adapter brick to generate the low voltages for operating controller ICs, light bulbs have limited space and cannot include a conventional AC/DC adapter. Instead, LED-based bulbs include a small switch-mode power supply that is configured to accept a high voltage AC line input to provide a DC low voltage output, much lower than the line input, to power the controller IC during normal operation. During the initial turn-on of the LED-based bulb, the DC low voltage output of the switch-mode power supply is unable to provide power for the controller IC and a start-up circuit is included with the switch-mode power supply to start the system and provide a temporary power supply while the primary supply is brought on-line.
FIG. 1 is one example of a conventional startup circuit 100 for an LED-based bulb in accordance with the prior art. The circuit 100 includes a high-voltage power field effect transistor (HV power FET) 112 coupled to an AC voltage input VIN node 102 through resistor 113. The HV power FET 112 is biased in saturation mode with a zener diode 122 and generates supply voltage VDD,H at node 106. The circuit 100 draws a high peak current from the VIN node 102 and continues to draw a small continuous current when the auxillary voltage input VAUX node is lower than the voltage VDD,H at node 106. The circuit 100 dissipates large amounts of power within the HV power FET 112, because of the continuous current draw, which at times is very high, and the bias condition of HV power FET 112. When the HV power FET 112 is integrated with other components into an integrated circuit (IC) for the LED-based bulb, the heat generated by the HV power FET 112 makes construction of the IC difficult. For example, the HV power FET 112 must be large enough to dissipate the heat without exceeding maximum power dissipation specifications. The larger size of the HV power FET 112 increases the cost of the IC and makes the IC difficult to incorporate to a light bulb.
FIG. 2 is another example of a conventional startup circuit 200 for an LED-based bulb in accordance with the prior art. The circuit 200 includes a depletion-mode power FET 212 coupled to an AC input voltage VIN node 102. The depletion power FET 212 is biased in saturation mode by zener diode 214 to generate a low voltage at VDD,H supply output node 106. Supply voltage VDD,H may be supplied to a controller to operate the controller 214 during start-up. Like circuit 100 of FIG. 1, circuit 200 continues to draw current from the VIN node 102 when the auxiliary voltage input VAUX node is lower than the voltage VDD,H at node 106. Although the depletion power FET 212 may only dissipate power during a transition time for the depletion power FET 212 to reduce power consumption, the circuit 200 is limited to low power applications where a required load current is less than approximately 0.5 milliAmperes. When load current is increased beyond 0.5 milliAmperes, power dissipation by the depletion power FET 212 creates similar problems to those described above in the circuit 100 of FIG. 1.
In both of the conventional start-up circuits described with reference to FIG. 1 and FIG. 2, the input voltage VIN node 102 is dropped across the power FET 112 or 212 when bias in saturation mode. Further, the output currents from the start-up circuits may be insufficient for starting up digital controllers, which may consume up to 5 or more milliAmperes. Thus, a circuit for providing larger load current with reduced power dissipation is needed.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved start-up circuit for low power dissipation, particularly for lighting devices and consumer-level devices. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art.